This invention relates generally to apparatus for measuring distance from the capacitance detected between a workpiece and a probe of the apparatus, and more particularly to grounding mechanisms in such apparatus.
Precise measurement of dimensional properties of a workpiece is of particular importance in the field of semiconductor wafer manufacture. The wafer must be held within tightly controlled dimensional specifications to be of the quality necessary for electronics applications. The critical dimensional specifications of the wafer include flatness, bow, warp and total thickness variation. Flatness is a measure of the roughness of one surface (or some smaller region thereof) of the wafer, and is expressed in terms of the sum of the largest positive and negative deviations of the surface from a reference plane. Bow relates to the convexity or concavity of the wafer, and is calculated from measurements taken on opposite sides of the wafer. Warp is the difference between the maximum and minimum distances of the median surface of the wafer and a reference plane, while total thickness variation is the maximum difference in thickness of the wafer.
It is well known in the industry of semiconductor manufacture to use capacitive distance measuring apparatus for accurately determining flatness, bow, warp and total thickness variation. Generally speaking such an apparatus includes a chuck which holds the wafer and is mounted on a shaft assembly for rotation, and for movement laterally and up and down. Probes are located at positions spaced from both the bottom and top sides of the wafer held on the chuck. The probes are capable of being excited to known electric potentials and act as one half of a capacitor, with adjacent surfaces of the wafer acting as the other half. Examples of probes and operating circuitry therefor are disclosed in U.S. Pat. Nos. 3,706,919, 3,775,678, 3,805,150 and 3,990,005, the disclosures of which are incorporated herein by reference.
Capacitive distance measuring requires that the wafer and the chuck be maintained at known potentials. Instantaneous fluctuations in voltage of the wafer or chuck will cause deviations in the measured capacitance so as to give a false distance reading. The wafer can be grounded (i.e., maintained at a "zero" potential), or electrically isolated so that its electric potential cannot change. The chuck is maintained at a constant potential by grounding. Grounding the chuck requires an electrical connection to be maintained between a stationary ground (i.e., a nearby portion of the apparatus) and the chuck or the shaft, as the chuck and shaft rotate. Presently existing ground contact mechanisms between the chuck or shaft assembly and the stationary ground employ solid electrical conductors spring biased into engagement with some portion of the shaft assembly. However in operation of the apparatus, momentary disconnections of the chuck from the stationary ground still occur causing voltage fluctuations in the chuck and false distance readings to be indicated by the apparatus.